The basic chromatography is the separation of components of a sample owing to their differences in solubility or in adsorption in a stationary bed of a material (either liquid or solid). When the sample (moving phase) is a gas, the technique is referred to as gas-solid or gas-liquid chromatography, depending on whether the stationary phase is a solid or a liquid. In gas chromatography, a sample is introduced into a carrier gas as a vapor which flows through a chromatographic system. Upon separation by the stationary phase, the analytes travel through the gas chromatograph at different speeds and enter a detecting device, which device is connected to the gas chromatograph, at different times. As a result, individual analytes that are present in the sample may be identified by the detecting device.
The analytes are transported using a carrier gas. The carrier gas is an inert gas for the analyte. Argon, helium and nitrogen are examples of carrier gases. Other gases and mixtures of gases can be used as well, depending on the implementations and/or the requirements.
A same gas chromatograph can be used with different kinds of detecting devices, depending on the needs. The various kinds of detecting devices can themselves have different sensitivity levels. For instance, some detecting devices can be designed to detect very low concentrations of an analyte, such as in the range of parts per million (ppm). Others can be designed to detect concentrations in the range of a few percent or more.
Some detecting devices can measure the concentrations of analytes based on ionization. The carrier gas with the analytes is directed from the outlet of the gas chromatograph to an ionization chamber located in-between a pair of electrodes provided inside the detecting device. The detecting device is designed to transform the carrier gas and each analyte into plasma using the electrodes. The plasma results in light radiations, including visible light. The light radiations can be sensed and recorded using one or more corresponding light sensors. The spectral content of the data obtained from the light sensor or sensors can reveal the presence of some analytes and their concentration.
Temperature fluctuations inside a detecting device can result in undesirable variations of the photons emissions. It is thus generally desirable to keep the detecting device at a substantially constant temperature during the data collection and at a temperature that is minimally equal to the temperature of the incoming gases. Incoming gases having a temperature lower than that inside the detecting device will be heated by the hotter inner surfaces of the detecting device and the impact of the temperature difference will be minimized. The temperature inside the detecting device can be maintained substantially constant using a heat source and a temperature monitoring system. However, incoming gases having a temperature higher than that of the detecting device will tend to increase the temperature inside the detecting device, thereby causing temperature variations during operation that are often difficult to control. For at least this reason, keeping the temperature inside a detecting device higher than the incoming gases is generally desirable.
Operating a detecting device at a relative high temperature can also mitigate carbon deposits on the inner surfaces of the ionization chamber. There are generally less carbon deposits when the operating temperature is increased and this can prolong the lifespan of the detecting device.
While operating a detecting device at a relatively high temperature can be a desirable goal, the added mechanical stresses imposed on the detecting device create additional challenges. These added mechanical stresses include thermal stresses caused by the thermal expansion of the various parts inside the detecting device when heated from the room (ambient) temperature to its operating temperature. The temperature variations will change the size of the parts, either when their temperature increases or decreases. In this context, the expression “thermal expansion” also refers to material shrinkage, for instance when the parts cool back to the room temperature once the detecting device is switched off.
A detecting device based on a Dielectric Barrier Discharge (DBD) configuration can include an air-tight housing made of a material such as quartz or the like. The electrodes provided around the ionization chamber to create the plasma discharge therein must be constantly maintained in position and the usual approach to achieve this goal is to add an adhesive, such as epoxy, between each electrode and the corresponding surface on the housing. A material such as quartz is relatively fragile and prone to cracking. In general, a housing made of quartz can tolerate the thermal expansion of the electrodes if the operating temperature is relatively low, for instance up to 70 degrees C., but often not at a relatively high operating temperature, for instance 300 degrees C., where the risks of damaging the housing are very high. A temperature such as 200 degrees C. is still a relatively high operating temperature in this context.
Another challenge is maintaining the integrity of the gas circuit inside the detecting device. Air leaks into the gas circuit can contaminate the gas samples being analyzed and affect the results of the measurements. The various connections are thus designed in effort that they will remain air tight and that their integrity will be preserved regardless of the temperature. Adhesives such as epoxy have been used in the past to seal the junctions going in and out of a detecting device.
However, when these adhesives are subjected to relatively high temperatures, they can release chemical compounds in the gas circuit and thereby contaminate the gas samples. The ionization chamber will then no longer be a highly pure environment.
Accordingly, there is still room for many improvements in this area of technology.